1. Field of the invention
The present invention relates to a piezoelectric micro energy harvester which converts mechanical energy generated in a surrounding environment to electrical energy and generates power by itself, and a method of manufacturing the piezoelectric micro energy harvester. More particularly, the present invention relates to a piezoelectric micro energy harvester, which has a small-sized simple structure and uses various kinds of materials as a piezoelectric film serving as a functional film for energy conversion, and a method of manufacturing the piezoelectric micro energy harvester.
2. Description of the Prior Art
In a general sensor, it is necessary to periodically change a battery in order for the sensor to receive power from a battery mounted inside the sensor, and at this time the entire sensor itself should be detached and attached, so that there occurs a problem in maintenance costs, life of a battery, influence by a high temperature, environment pollution, etc. In this respect, the necessity of a self-powered sensor generating electrical power by itself and operating, instead of external power including a battery or power source in a home, has increased, and especially, a development of an energy harvester (or energy scavenger) capable of supplying power to a wireless sensor has been demanded.
Therefore, it is prospected that wireless sensor technology enables the sensor to be developed as a batteryless type sensor requiring no fixed battery serving as power sources. Further, the technology is applied to transportation and logistics, for example, a Tire Pressure Monitoring System (TPMS), a factory management including the motor state control, the management of a power network including smart grid, smart home and building control, environment field, and agricultural and fishery business, so that the relevant services can be more technologically enhanced.
However, the development of an energy harvester, which is a fundamental solution for operating a sensor module without power in various environments of various places, has been unfinished, and international advanced research groups are conducting research on the application of vibration, sun, wind, heat, etc. as power resources. The wireless sensor module receives necessary power from a physical quantity of such power resources so as to extend an operational time or is developed as a buried sensor type for specific usages, so that it is possible to provide more beneficial information.
Examples of places or apparatuses providing an environment including vibration all the time include a motor, a rotational device of automobile tires, etc. A wireless sensor module monitoring a state of the motor or the rotational device of the automobile tires is installed together with the energy harvester. The energy harvester converts mechanical vibration to electrical energy, so that it is possible to provide power to the wireless sensor module without an outer power supply device.
The energy harvester, which converts vibration, impact, rotational force, inertial force, pressure, fluid flow, etc. generated in a surrounding environment to electrical energy employs piezoelectric conversion, electromagnetic induction, electrostatic conversion, etc. serving as a conversion mechanism. Among the conversion mechanisms, the piezoelectric conversion is a method using a piezoelectric material as an energy conversion functional material and uses a property that when a strain of the piezoelectric material consisting of an inorganic material, such as ceramic, or an organic material, such as polymer, is changed, an electrical charge is generated. Therefore, the piezoelectric conversion method is advantageous in that the conversion method is simple, it is possible to obtain a high output voltage, and the outer voltage resources are not necessary so that a structure of the method be easily implemented.
The piezoelectric energy harvester using the aforementioned piezoelectric material includes a piezoelectric structure and electrodes and collects electric charges generated in accordance with the change of the mechanical strain applied to the piezoelectric structure by using the electrodes, so that it generates electrical energy by itself.
A conventional piezoelectric energy harvester has been usually implemented by a method in which a sintered ceramic piezoelectric material is cut in a patch shape and is then attached to a mechanical structure that is mechanically movable, or by a method in which a piezoelectric material in a form of a thick film is formed on a material having a relatively low stiffness including FR-4-based PCB (Printed Circuit Board), a polymer material, PDMS (Polydimethylsiloxane), etc. However, according to these methods, the structures in various forms are mechanically processed, arranged, and assembled, so that there is a problem of the increase of the manufacturing costs.
In the meantime, a small-sized piezoelectric micro energy harvester, which mainly uses Micro Electro Mechanical System (MEMS) technology employing a semiconductor manufacturing process, has been recently researched. However, the piezoelectric micro energy harvester uses a minimum of three or four pattern masks and a maximum of above ten pattern masks for forming of main functional elements. The piezoelectric micro energy harvester is manufactured by repeatedly performing the process steps of disposing a thin film, coating a photo resist film, and micro-patterning, and etching the thin film and then sequentially forming the functional elements in a vertical direction of a substrate. Therefore, in the event of manufacturing the piezoelectric micro energy harvester by using the conventional semiconductor manufacturing process, there is a problem of the large manufacturing costs, long manufacturing time, and the decrease of a manufacturing yield.
According to the conventional method of manufacturing most of the piezoelectric micro energy harvesters, the main functional elements, such as the electrodes and the piezoelectric film, are sequentially formed on a silicon substrate by a micromachining process of a bulk micromachining, a back surface of the silicon substrate is subjected to the micromachining process so as to form etched pits or grooves, and forming a suspended structure shaped like a cantilever separated from the substrate together with proof mass. Such a method is a process of processing front and back surfaces of a substrate, so that the process requires a large cost and is generally implemented by a wet etching using a crystalline orientation of the silicon substrate itself, so that it is impossible to optionally control a geometric form, such as a depth and a shape of a microstructure of the piezoelectric micro energy harvester, thereby resulting in the limitation of the miniaturization of the piezoelectric micro energy harvester. Further, in implementing the structure of the piezoelectric micro energy harvester by using a high-priced substrate, such as the SOI (Silicon-On-Insulator) structure, the manufacturing costs of the piezoelectric micro energy harvester further increases.
In the meantime, the piezoelectric micro energy harvester requires a functional material for energy conversion, which converts mechanical energy input from an outside environment to electrical energy, and mainly uses a piezoelectric material in the event of the piezoelectric scheme.
For example, the piezoelectric material including an inorganic material, such as ceramic including PZT (Lead Zirconate Titanate, PbZrxTi1-xO3), PMN-PT [(1-x)Pb(Mg1/3Nb2/3) O3-xPbTiO3], BaTiO3, ZnO(Zinc Oxide), or AlN(Aluminum Nitride), metal oxide, and a semiconductor, an organic material, such as PVDF (Polyvinylidene Fluoride), and a nano material, such as nano wires or nano tubes has been researched. A conventional piezoelectric film is formed by directly micro-patterning a material of the piezoelectric film on devices by using a pattern mask during a manufacturing process of the piezoelectric micro energy harvester, or cutting a raw material in an original material state in a patch shape and then precisely aligning devices and bonding the cut material to corresponding positions of surfaces of the devices. For these methods, it is necessary to develop a unique forming process and bonding method according to each of the piezoelectric film materials to be used and the alignment equipment is required, so that the manufacturing costs is disadvantageously high. Therefore, in order to use various kinds of materials as the material of the piezoelectric film serving as the energy conversion functional film, technology for easily forming the microstructure at a specific position of surfaces of the devices is demanded.
In the meantime, mechanical vibration frequency spectrums of various bands exist in nature and it is extremely difficult to individually control the spectrums, so that the mechanical energy has to be converted to the electrical energy even in the various external mechanical frequencies in order to actually implement the piezoelectric micro energy harvester. That is, it is necessary to design the piezoelectric micro energy harvester so that the piezoelectric micro energy harvester is adequate for the broadband frequency or it is necessary to minutely analyze a characteristic of a frequency of an outside environment and design the piezoelectric micro energy harvester in detail so that the piezoelectric micro energy harvester corresponds to the analyzed frequency characteristic. However, it may be actually more ideal to design a structure of the piezoelectric micro energy harvester capable of responding to the outside broadband frequency in an aspect of the energy efficiency.
Therefore, the piezoelectric micro energy harvester having the advantages that it has a small-sized simple structure, can be simply manufactured to reduce the manufacturing costs, can be implemented in any shape, can easily use various kinds of piezoelectric film as a energy conversion functional film, and can respond to the outside broadband frequency, and a method for manufacturing the piezoelectric micro energy harvester have been demanded.